A dual fuel engine is defined herein to be an engine that can be fuelled with two different fuels at the same time, whereas a bi-fuel engine is defined herein to be an engine that can be fuelled with either one fuel or another fuel. It is possible for some engines to be operated as a dual fuel engine under some conditions and as a bi-fuel engine under other conditions. Due to several factors, engines fuelled with natural gas as a supplementary or alternative fuel in dual-fuel and bi-fuel engines are becoming more common. For example, recent advances in natural gas fuelling systems have allowed some dual-fuel engines that inject natural gas at high pressure directly into the combustion chamber to match the performance characteristics of diesel fuelled engines. Emission reductions obtained by substituting natural gas for diesel is allowing engine manufacturers to meet ever more stringent emission standards. Other factors include escalating diesel and gasoline fuel costs, and concerns over energy independence. As a result, many engine manufacturers are adapting directly injected, diesel cycle engines to substitute natural gas for diesel fuel.
Natural gas fuelling systems can store natural gas as either liquefied natural gas (LNG) or compressed natural gas (CNG). For vehicles, the use of LNG extends the driving range because the higher energy density allows more fuel to be stored in the same storage volume but also adds complexity to the fuel storage system, and so is used typically on larger vehicles that require extended operating range, such as commercial transport trucks. Regardless of the form in which natural gas is stored, at some point it is in a gaseous phase at low pressure and requires pressurization to the desired pressure for injection into combustion chambers through a fuel injector. A high pressure, gaseous fuel compressor is employed, such as the pump described in the applicant's own U.S. Pat. No. 7,527,482 (the '482 patent), to raise the pressure to the level required for gaseous fuel injection.
The compressor disclosed in the '482 patent is of the reciprocating piston-type. It comprises a hollow cylinder and a reciprocal free-floating piston disposed therein. The piston divides the cylinder into a compression chamber within which a gas can be introduced, compressed, and discharged; and a drive chamber, into which a hydraulic fluid can be introduced and removed for actuating the piston. The ratio of the area of a driving face of the piston on the driving chamber side to the area of a compression face of the piston on the compression chamber side is 1:1.
To run the compressor in preferred embodiments the hydraulic fluid is provided by a pre-existing hydraulic system on the engine that has enough capacity to additionally run the compressor. Vehicles typically have pre-existing hydraulic systems that include a hydraulic pump that is mechanically driven by attachment to a power take off which is ultimately driven by the engines crankshaft. However, with some engines there is not enough additional capacity in pre-existing hydraulic systems and either a higher capacity hydraulic pump must be installed or an additional hydraulic pump must be added if there is room for an additional power take-off. In some cases a reservoir needs to be added for the new or upgraded hydraulic pump, in addition to components for cooling the hydraulic fluid and for distributing the hydraulic fluid to the pump.
Modern day diesel cycle engines typically employ a liquid-fuel common rail direct injection system. Fuel pressure in a common rail is typically above 160 MPa, and advances in diesel system fuel technology allow pressures to reach as high as 200 MPa.
High injection pressures are required to atomize the diesel liquid fuel into tiny droplets as it is injected into the combustion chamber. Heat from hot compressed air then acts to vaporize the fuel from the surface of the droplets. The vapor is then ignited due to the heat of the compressed air, the droplets continue to vaporize from their surface and burn, getting smaller, until all the fuel from the droplet has been vaporized and burnt. Additionally, the injection pressure must be high enough to overcome in-cylinder pressures encountered when the fuel injector valves are actuated. The compression ratios are high for modern diesel cycle engines and typically can be in the range of 15:1 to 22:1. Normally, fuel injection begins at or near top dead center during the compression stroke, and can also occur in the power stroke. When fuel injection begins in-cy Under pressures before ignition can be at least as high as 4 MPa, and can quickly rise as combustion commences.
In order to achieve the desired injection pressures, a high-pressure diesel fuel pump is employed to raise the pressure of the diesel fuel to the common rail pressure. The term “common rail” describes an arrangement whereby all of the fuel injectors are supplied by a common fuel-rail which acts as a fuel distribution manifold and a pressure accumulator where the fuel is stored at high pressure. The common rail supplies multiple fuel injectors with high pressure fuel. This simplifies the design of the system and the high pressure pump since it only has to maintain a single commanded pressure at a target. Since diesel fuel is an incompressible fluid it can be brought to the required pressure quickly and with relatively little energy.
Diesel-cycle engines adapted for use with natural gas fuelling systems require an ignition source for the gaseous fuel. The natural gas auto-ignition temperature is approximately 580° C., which is significantly higher than the diesel fuel auto-ignition temperature of approximately 210° C. The maximum temperature of the compressed air charge inside a diesel-cycle engine with a compression ratio less than 22:1 is under 550° C. Diesel fuel can be employed as a pilot fuel wherein a small of amount of diesel is injected into the combustion chamber after the natural gas is injected in order to initiate combustion. The amount of diesel fuel that is consumed as a pilot fuel is typically less than 20% and in preferred embodiments averages less than about 5% of the total fuel consumed on an energy basis.
For engines and vehicles that do not have room for an additional power take off and for a dedicated hydraulic pump there is a need for a new and improved method and apparatus for actuating a liquid pump or gas compressor, used to pressurize a gaseous fuel for high pressure direct injection into the combustion chambers of a dual-fuel or bi-fuel engine.